Ensure that each thread has its own client

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Implements the fix suggested by @jurriaan in #590. I don't have a lot of experience with threads in Ruby but this seems reasonable.

(Had to update .rubocop_todo.yml because the new test caused a Metrics/ClassLength offense. We might want to exclude test code from those metrics cops.)

[brandur-stripe]

Thanks Olivier!

I think there is also a chance that we should think about moving to something besides Faraday on a future major release — as @jurriaan mentioned, it doesn't do connection pooling by default (or even easily), and this is really unfortunate given that a lot of people aren't going to think to turn it on. We probably have a huge number of Europe-based users who'd hugely benefit from a few tweaks in this area.

For now though, this makes sense.

[jurriaan]

This is awesome! Thanks a lot!